The word Stupid is a misnomer. The modern definition of stupidity refers to a lack of intelligence. While this may seem like an extreme, it is a relatively easy entry point. Having no intelligence doesn't mean you cannot be successful in life, either. The definition of stupidity is vast and can cover anything from a single Facebook post to mass demonstrations. The world is full of these people, so it's not just an issue of poor education.
If we were to look at the US population, we would see that a large segment of the population has an IQ of ninety or less. That's an alarming statistic because a population with lower IQ scores has a more challenging time getting a decent job, and a higher IQ is correlated with greater financial security. So even if an intelligent person doesn't want to work for a fortune, a low IQ will not prevent them from getting a good job and a high-paying one.
The SAT is a perfect example. The less intelligent population has less tendency to be arrogant, biased, and anxious. They're also less likely to assume recent trends will continue. They're also more likely to be racist and to have an inability to understand the truth. Those who are a little more intelligent are more likely to be tolerant and open-minded. It's not that you shouldn't argue with such people - they are just ignorant.